1. Field of the Invention
This invention relates to seismic sound sources for use on land. In particular, it is concerned with imploders.
2. The Prior Art
Seismic sound sources, for use on land, customarily are designed to impart a compressional acoustic wave into the earth whose first motion is downwards in a pushing motion. As is well known, longitudinally-propagating seismic waves (technically known as compressional or P-waves), generate alternate compressional and rarefactional stresses on the elastic earth. All known prior-art sound sources used on land, generate wave forms whose first motion is compressional. The seismic literature is replete with descriptions of such sources including explosives, thumpers (weight-drop systems such as hammers or the like) underwater sparkers that are discharged in water-filled holes in the ground, air guns and gas guns.
In the matter of vehicle-mounted air guns, a large tank of water is provided. Two or three air guns are mounted in a single, closed tank of water. The tank is pressed against the ground by the combined weights of water itself and that of the transporting vehicle. Air under several thousand pounds is admitted to the gun. When the air is discharged, a powerful compressional or P-wave is suddenly generated, whose initial phase is compressional. The inertia of the water mass serves as a reaction mass to more efficiently transfer the acoustic signals into the ground.
Although the air-gun-water-tank system is known and used commercially, it has many disadvantages. First, because of the very high air pressures, 2000 to 6000 psi, that are needed, a large air compressor is required. Usually rated at 80-120 ft.sup.3 /min the compressor requires an auxiliary drive engine of up to 80 HP. Additionally, because the high-pressure air must be vented from the water tank, an elaborate set of baffles is necessary to keep the water from blowing out of the tank after each shot. Further, the downward compressional force adversely compacts the ground, particularly if it is soft, and leaves undesired depressions thereon.
Typical prior-art hydraulic signal generators similar to the one discussed above, are described in U.S. Pat. Nos. 3,310,128; 3,779,335; 3,800,907, all to Chelminski, and U.S. Pat. No. 4,008,784 to Bays.